This invention relates to an organic electroluminescent (EL) device, and more particularly, to a compound for use in a device of the type wherein an electric field is applied across a thin film of an organic compound to emit light.
Organic electroluminescent (EL) devices include a thin film containing a luminescent organic compound interleaved between an electron injecting electrode and a hole injecting electrode. Electrons and holes are injected into the thin film where they are recombined to create excitons. Light is emitted by utilizing luminescence (phosphorescence or fluorescence) upon deactivation of excitons.
The organic EL devices are characterized by plane light emission at a high luminance of about 100 to 10,000 cd/m2 with a voltage of about 10 volts and light emission in a spectrum from blue to red color by a simple choice of the type of fluorescent material.
Doping is one technique for producing light emission of any desired color from EL devices. It was reported in Jpn. J. Appl. Phys., 10, 527 (1971) to change emission color from blue to green by doping anthracene crystals with a minor level of tetracene. With respect to organic thin film EL devices having a multilayer structure, it was reported in JP-A 63-264692 to incorporate in a host material having a light emitting function a minor amount of a fluorescent dye capable of emitting light different from that of the host material in response to light emission from the host material as a dopant to form a light emitting layer, thereby changing the color of light emission from green to orange or red.
With respect to long wavelength light emission of yellow to red, known light emitting materials or dopant materials include laser dyes capable of red oscillation (EP 0281381), compounds capable of exciplex emission (JP-A 2-255788), perylene compounds (JP-A 3-791), coumarin compounds (JP-A 3-792), dicyanomethylene compounds (JP-A 3-162481), thioxanthene compounds (JP-A 3-177486), mixtures of a conjugated polymer and an electron transporting compound (JP-A 6-73374), squalirium compounds (JP-A 6-93257), oxadiazole compounds (JP-A 6-136359), oxynate derivatives (JP-A 6-145146), and pyrene compounds (JP-A 6240246).
Other light emitting materials disclosed heretofore include condensed polycyclic aromatic compounds (JP-A 5-32966 and 5-214334). Also dopant materials proposed heretofore include various condensed polycyclic aromatic compounds (JP-A 5-258859).
These light emitting systems, however, do not provide high luminance or stable light emitting performance, especially at long wavelength. A further improvement in luminance or durability is thus desired.
An object of the invention is to provide a compound for use in an organic EL device and an organic EL device which produce light emission of satisfactory luminance, especially at long wavelength and are durable in that improved light emitting performance lasts for a long time.
This and other objects of the invention are attained by the following construction.
(1) A compound for organic electroluminescent devices, having a basic skeleton represented by the following formula (I): 
wherein R1, R2, R3, and R4 each are a substituted or unsubstituted aryl or alkenyl group; R5, R6, R7, and R8 each are hydrogen; at least two of R1 to R4 are aryl groups which are at least bicyclic, aryl groups having aryl, amino, alkenyl, aryloxy or heterocyclic groups as substituents, or alkenyl groups, and when at least two substituents are monocyclic aryl groups, they further have substituents, and when the substituents are amino groups, at least two of R1, R2, R3, and R4 have substituted or unsubstituted aryl groups as substituents.
(2) The compound for organic electroluminescent devices of (1), having a basic skeleton represented by the following formula (II): 
wherein each R group in sets of R11 to R15, R21 to R25, R31 to R35, and R41 to R45 is hydrogen or a substituted or unsubstituted alkyl, aryl, amino, heterocyclic or phenoxy group, the R groups in at least two sets have substituted or unsubstituted aryl, heterocyclic or aryloxy groups as substituents, or when the R groups are all hydrogen, at least two R groups in each of the sets of R11 to R15, R23 to R25, R31 to R35, and R41 to R45 form a fused ring.
(3) The compound for organic electroluminescent devices of (1), having a basic skeleton represented by the following formula (III): 
wherein X represents a substituent as defined for the substituents on R1, R2, R3 and R4, and i is an integer of 0 to 20.
(4) An organic electroluminescent device comprising a hole injecting electrode, an electron injecting electrode, and an organic layer disposed between the electrodes and including at least a light emitting layer, wherein
said light emitting layer contains the compound set forth in any one of (1) to (3).
(5) The organic electroluminescent device of (4) wherein said light emitting layer further contains an electron injecting and transporting compound and/or a hole injecting and transporting compound.
(6) The organic electroluminescent device of (4) or (5) wherein said light emitting layer contains at least two compounds.
(7) The organic electroluminescent device of (4) or (6) wherein said light emitting layer contains at least two dopants, the content of the dopants combined being up to 30% by weight based on a host material.
(8) The organic electroluminescent device of any one of (4) to (7) wherein the overall content of the compound of (1) in said light emitting layer is up to 30% by weight based on a host material.
(9) The organic electroluminescent device of (7) or (8) wherein said light emitting layer contains at least two compounds having different carrier trapping capabilities.
(10) The organic electroluminescent device of any one of (7) to (9) wherein said light emitting layer contains at least a compound having a hole trapping capability and a compound having an electron trapping capability.
(11) The organic electroluminescent device of any one of (4) to (10) wherein at least two light emitting layers are included, and said light emitting layers contain dopants having different carrier trapping capabilities.
(12) The organic electroluminescent device of any one of (4) to (11) wherein at least two light emitting layers are included, at least one layer of said light emitting layers contains a dopant having a hole trapping capability, and at least one other layer of said light emitting layers contains a dopant having an electron trapping capability.
(13) An organic electroluminescent device comprising a hole injecting electrode, an electron injecting electrode, and an organic layer disposed between the electrodes and including at least a light emitting layer, wherein
said light emitting layer contains at least two organic electroluminescent device-forming compounds having different carrier trapping capabilities, each said compound having a basic skeleton represented by the following formula (IV): 
wherein R101, R102, R103, and R104 each are hydrogen or a substituted or unsubstituted aryl or alkenyl group, excluding the case where at least three R""s are hydrogen atoms; R105, R106, R107, and R108 each are hydrogen or a substituted or unsubstituted aryl or alkenyl group; at least two of R101 to R104 are aryl groups which are at least bicyclic or alkenyl groups, or have alkyl, aryl, amino, alkenyl, aryloxy or heterocyclic groups as substituents.